The selection of polymers in particular application depends both on the polymer molecular weight and on the molecular weight distribution. Very often it is necessary to control the molecular weight of the polymer so that it may be fitted to its particular use. Polymers having low molecular weight with some functionality referred as macromers have become of increasing interest since they are useful in a variety of applications such as surface-active agents, dispersants, binders and coatings, sequestering and sealing agents. In this respect, water-soluble oligomers and low molecular weight polymers of acrylic acid (AA) of average molecular weight up to 20,000 appear to be especially useful in various applications. A typical application of polyacrylic acid (PAA) is in the thickening of natural rubber latices. The addition of low molecular weight PAA as the sodium salt to 40 wt. % natural rubber as a creaming agent causes separation of the system into two layers, i.e. clear latex serum and concentrated latex containing 60 wt. % rubber. If low molecular weight PAA solution is sprinkled over propylene sheet to which aluminium foil has to be stuck, the resulting AI-PP laminate has a high mechanical strength. The optimum PAA molecular weights for application as dispersing agents in water energy systems lie in the range 1,000-4,000. Poly(acrylic acid) with Mn values in the range 10,000-18,000 may be used for dispersing mineral fillers, inorganic pigments and other minerals in water systems to obtain stable suspensions or slurries suitable for pumping. For example, the separation of kaolin from deposits is best effected by a polymer with Mn in the range 4,000 to 10,000 using AA polymer content in a 0.001-2.0 wt % ratio in relation to the dry mineral. Another field of application for low molecular weight PAA is as a dispersant for the improvement of binding agents such as cement and gypsum in water slurries. It also accelerates concrete setting (quick-setting concretes). Acrylic acid polymers, ranging in molecular weight from the lowest oligomers up to 20,000 may be employed for talc purification (e.g. 0.1-2 wt. % pAA). (Spychaj T., Progress in Organic Coatings, 17, 1989, 71)
Low molecular weight Poly(vinyl alcohol) (PVA) has been used for many industrial applications, e.g. paper-coating, fibre-sizing, as a stabilizer in disperse systems, and manufacture of fibre and film. It is often used in the blends of other polymers to improve its solution and bulk properties. Some polymers such as polyvinylpyrorrlidone and cellulose and its derivatives are known to be miscible with PVA in the blend films due to some specific interactions, e.g. hydrogen bonding between PVA and these polymers. However, many other polymer blends containing PVA are known to exhibit macroscopic phase separation. On the other hand, in order to prepare graft copolymers with a well-defined structure, one can use the macromonomer technique. Block and graft copolymers containing the PVA sequence as one component have been used as compatibilizers for the blends to attain fine dispersion, which makes various properties of the blends better. Recently, the free-radical polymerization of VAc with chain transfer agents was applied to the preparation of the block copolymers containing the PVA sequence (Ohnaga T., Sato T., Polymer 37(16), 1996, 3729). Also, the copolymers prepared from macromers have different properties and hence may also open up new areas of usability. AA-St (acrylic acid-styrene) copolymers containing 40 wt % AA and with molecular weights within the range 400-4,000 are used as water-soluble lacquers for paper. Low molecular weight AA-St copolymers provide as excellent basis for the formulation of inks for printing on paper, polymer or metallic foils. (Spychaj T., Progress in Organic Coatings, 17, 1989, 71)
Examples of low molecular weight AA-St resin usage in the paint industry include water-proofing sealants for roofs and conductive coatings based on powdered aluminium filler as well as mould-resistant emulsion paints with a high performance (Kapse G. and Aggarwal L., Paint India, 33, 1983, 5).
A characteristic example of the application of St-AA copolymers is in self-polishing floor coatings on linoleum or poly(vinyl chloride) plates. These coating materials are water-soluble in the presence of ammonia, but after drying (and partial ammonia evaporation) become insoluble. Other advantages exhibited by such self-polishing floor coatings are resistance to detergents, abrasion and soiling, and the existence of a high polish over a long time span. The above requirements are best fulfilled by AA-St copolymers in which the comonomer mole ratio is 1:2 and with molecular weights ranging from 500 to 6,000. (Spychaj T., Progress in Organic Coatings, 17, 1989, 71)
Using a chain transfer agent in free radical polymerization can reduce the polymer molecular weight by its chain-breaking action. Organic compounds such as mercaptans or alkyl bromides have been widely used in polymerization processes to control polymer molecular weight. U.S. Pat. No. 4,000,220 discloses the use of chain transfer agent such as mercaptans, thiopropionic acid, carbon tetrachloride and dimeric alpha methylstyrene in the production of thermoplastic graft copolymer resins where introduction of small amount of double bonds i.e. AMSD increases the weatherability and hence impact strength of the resin. U.S. Pat. No. 4,001,349 discloses the use of chain transfer agents such as mercaptans for the preparation of grafted products of styrene and saturated polyolefinic elastomers. U.S. Pat. No. 4,427,826 discloses polymerizing 1,3-diene rubber and one or more vinyl monomers, with or without a solvent, in the absence of a free radical initiator and in the presence of a mercaptan chain transfer agent. Macromers with unsaturation can also be obtained using chain transfer agents such as cobalt (II or III) chelates as disclosed in U.S. Pat. No. 4,680,352 and U.S. Pat. No. 4,694,054. The use of terminally ethylenically unsaturated oligomers as chain transfer agents, for controlling the molecular weight of certain polymers is also known. Such oligomers are known, for example, as disclosed in U.S. Pat. No. 4,547,327, U.S. Pat. No. 4,170,582; U.S. Pat. No. 4,808,656.
Free-radical copolymerization of a macromonomer with a vinyl or an acrylic comonomer has been and is still the major field of macromonomers because it provides easy access to graft polymers. Macromonomers bearing unsaturated end groups (e.g., 2-substituted-2-propenyl end groups) that are reactive toward addition of propagating radicals of monomers such as methacrylates, acrylates, and styrene (St) have attracted attention as useful precursors for synthesis of branched or graft polymers by conventional free radical polymerization. Macromonomer synthesis by conventional radical polymerization, and the reactions of these macromonomers, have been widely studied in recent years. The approach has the advantage of its moderate conditions compared to living ionic polymerizations.
Changing the chemical structure of the polymer chain can provide macromonomers having various properties, while changing the comonomer can lead to polymers having different properties. At the same time, macromonomers have some advantages such as non-volatility and high solubility that are different from those of small monomers and polymers, which make it easy to control them in further reaction. So, design and synthesis of macromonomers with various structures is useful in developing new polymeric materials.
The synthesis of macromonomers is mainly by two methods. The first one, called the end-capping agent. The second one, called the initiation method, utilizes an unsaturated initiator to bring about polymerization of monomer to form macromonomer directly. These two methods have been successfully applied in synthesis of macromonomers via anionic, cationic, or group-transfer polymerization. The harsh conditions and limitation in choice of monomer for ionic living polymerization, there are many attempts for preparation of macromonomers by radical polymerization.
Catalytic chain transfer (CCT) polymerization is one of the most effective method to prepare macromonomers in radical polymerization. The polymerization of acrylates and St at high temperature has been also shown to yield macromonomers via formation of midchain radicals followed by fragmentation. However, effective CCT polymerization resulting in carbon-carbon double bonds is restricted to the homopolymerization and copolymerization of R-methylvinyl compounds such as methyl methacrylate (MMA) and R-methylstyrene, (Sato E., Zetterlund P. and Yamada B., Macromolecules, 37, 2004, 2363).
Although a prominent advantage of free radical polymerization is the tolerance to electrophilic and nucleophilic compounds, in particular to the presence of water, controlled free radical polymerization studies in aqueous solution are minority. This is due to the need of modifying the necessary additives to give water-solubility to them. And also this is due to the high temperatures of above 100° C., as often needed for nitroxide mediated polymerization (NMP), or to the sensitivity of the ‘controlling agents’ to the presence of water, as for many atom transfer radical polymerization (ATRP) catalysts. In this context, the use of the reversible addition fragmentation transfer (RAFT) method appears particularly appealing for aqueous polymerization systems. Still, the number of reports on the use of the RAFT method in aqueous systems is limited. However, the commonly used classes of dithioester and trithiocarbonate compounds are known to be sensitive to hydrolysis. (Baussard J., Habib—Jiwan J., Laschewsky A., Mertoglu M., Storsberg J., Polymer 45, 2004, 3615
In conventional radical polymerization, the macromonomer precursor method is very useful; the macromonomer precursor was synthesized at first by using an appropriate transfer agent such as thioglycolic acid, then the unsaturation was introduced by the reaction. But, use of this method requires two steps. Hence, there is a need to obtain unsaturated macromers, which can be used directly for copolymerization with any vinyl monomer.
Meijs et. al. (Meijs G., Rizzardo E. and Thang S., Macromolecules, 21, 1988, 3122, Meijs G. and Rizzardo E., Makromol. Chem., 191, 1990, 1545, Meijs G., Morton T., Rizzardo E., and Thang S., Macromolecules, 24, 1991, 3689) have reported that allylic compounds activated by phenyl, alkoxy carbonyl and cyano groups undergo chain transfer via sequential radical addition and fragmentation reactions which gives polymer with terminal double bond.
2,4-diphenyl-4-methyl-1-pentene, i.e., α-methylstyrene dimer (α-MSD), is also known to be an effective chain transfer agent for styrene polymerization. S. Suyama et al has reported the addition—fragmentation chain transfer in free radical styrene polymerization in the presence of 2,4-diphenyl-4-methyl-1-pentene. They have proposed, a chain transfer mechanism through addition—fragmentation reaction. (Watanabe Y., Ishigaki H., Okada H. and Suyama S., Chemistry letters, 1993, 1089) That is, polymer radical adds to the terminal double bond of α-MSD and then the adduct radical undergoes fragmentation to give a cumyl radical and a polymer with a terminal double bond. This is because cumyl radical being tertiary radical is more stable than polymer radical being a secondary radical. The report on the use of AMSD as a chain transfer agent for hydrophobic monomers such as methyl methacrylate, styrene is known. It also tells that the use of AMSD in free radical polymerization gives the functionality to the polymers in terms of terminal unsaturation.
Fischer and Luders (Fischer J. and Luders W., Makromol. Chem., 155, 1972, 239,) have proposed that the chain transfer would proceed through allylic hydrogen abstraction by polymer radical and/or addition of polymer radical to α-MSD and subsequent hydrogen atom transfer to styrene. Being α-MSD very hydrophobic in nature not applicable in aqueous polymerization systems. There are numerous applications of water-soluble oligomers having terminal unsaturation.
The use of equimolar amounts of cyclodextrins to dissolve suitable hydrophobic monomers in water and the free radical polymerization of such host/guest complexes has recently been investigated by Ritter et. Al. (Jeromin J., Noll O., Ritter H., Macromol. Chem. Phys. 199, 1998, 2641, Jeromin J., Ritter H., Macromol. Rapid Commun. 19, 1998, 377, Jeromin J., Ritter H., Macromolecules 32, 1999, 5236, Glockner P., Ritter H., Macromol. Rapid Commun., 20, 1999, 602, Glockner P., Metz N., Ritter H., Macromolecules 33, 2000, 4288, Ritter H., Storsberg J., Pielartzik H., Groenendaal L., Adv. Mater. 12, 2000, 567).
The use of a catalytic level of cyclodextrin allows the use of very hydrophobic monomers in emulsion polymerization where cyclodextrin acts as a phase transport catalyst continuously complexing and solubilizing the hydrophobic monomers and releasing them to the polymer particles. (Lau W., Macromol. Symp. 182, 2000, 283-289, Leyrer R., Machtle W., Macromol. Chem. Phy., 201, 2000, 1235-1243)
The chain transfer constants for 1:1 host-guest complexes of methyl methacrylate-me-β-CD and styrene-me-β-CD were determined using water soluble dodecanethiol-me-β-CD complex. The chain transfer constant of this complexed system was found to be lower when compared with the uncomplexed system. (Glockner P., Ritter H., Macromol. Chem. Phy., 201, 2000, 2455-2457).
Investigation on free radical polymerization of CD complexed monomers in the presence of N-acetyl-L-cysteine as hydrophilic chain transfer agent in aqueous medium is reported by Ritter et. Al. Relatively high chain transfer constants of N-acetyl-L-cysteine were found in the case of the complexed methyl methacrylate and styrene monomers in water. (Glockner P., Metz N., Ritter H., Macromolecules, 33, 2000, 4288).
Up to now only two reports on chain transfer activity of mercaptans on the degree of polymerization of CD—complexed monomers has been evaluated. But, in these reports, only the effect of steric hindrance on the chain transfer activity of the chain transfer agents was investigated.
α-methyl styrene dimer in organic medium has been effectively used as a chain transfer agent as reported in the literature. But, there is no report on the use of AMSD as a chain transfer agent in aqueous medium. The survey of the prior art in the field of utilization of chain transfer agent reveals that the use of AMSD-DM-β-CD complex for water soluble monomers in aqueous system giving water soluble macromers having terminal unsaturation has not been reported till date. The terminal unsaturation obtained using AMSD-DM-β-CD complex has a reasonable reactivity in copolymerization. It has been found that AMSD forms an inclusion complex with DM-β-CD and becomes water-soluble and hence can be applied as a water-soluble chain transfer agent. Further, the chain transfer activity of the agent is not reduced and also it incorporates unsaturation as a terminal functionality. This property is used for the polymerization of various water-soluble monomers, which gives water-soluble macromers having unsaturation as an end-functionality. The terminal unsaturation can be used further for crosslinking or grafting of any comonomer depending on the particular application.